This invention pertains to the art of manufacturing valve sleeves and more particularly to a tool and method of coining chamfers or corner breaks for oil grooves formed in the valve sleeve.
The invention is particularly applicable to forming corner breaks in a stator or valve sleeve of a power steering valve as employed in an automotive vehicle and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other apparatus wherein a coining operation is required on an internal surface of a sleeve.
The tool for and method of coining an internal surface according to the subject invention are particularly applicable to the sleeve portion of a power steering valve, which steering valves are well known in the art. Rather than describe in detail the structure and operation of a conventional power steering valve, reference may be had to U.S. Pat. No. 4,516,471 issued to Duffy on May 14, 1985. That patent provides a detailed description of the environment for which the tool and method of coining is particularly applicable.
The noted patent is directed to providing chamfered lands on a rotor portion of a steering valve. The schematic representation of FIGS. 6 and 6A of that patent clearly demonstrate the chamfered regions on the rotor. Additionally, the valve sleeve of that patent is typically broached along its inner surface to form the necessary oil grooves. Chamfers are also formed at the corners of the groove during this broaching operation. This type of structure, though, necessarily requires use of end rings inserted at opposite ends of the valve sleeve in order that a sealed arrangement may be provided with the rotor. Further, the chamfers are formed on only one of a pair of opposed corner breaks defined by each groove. Prior art arrangements of this type require a significant amount of machining as well as the use of plural components to join together the sleeve and rotor.
One solution for the open ended slot arrangement resulting from a broaching operation on the rotor is described in U.S. Pat. No. 4,614,014 issued to Ferguson on Sept. 30, 1986. According to that method, a plurality of open-ended longitudinal slots are cut or broached into the valve sleeve and then end portions of the slots are wedged to deform the slot portions and form spaced and discontinuous protrusions that block the open-ended slots. Although this method has met with substantial success, it does not resolve the persistent problem of providing accurately positioned chamfers along the corner breaks of the oil grooves.
Typically, a valve sleeve is drilled or bored and the inner diameter tapped as necessary. The valve sleeve may be heat treated and is diamond-honed for particular sizing with an associated rotor. It is believed that a sliver burr is formed at the corner breaks of the oil grooves of the valve sleeve resulting from material being pushed into the groove during the diamond-honing operation. These burrs restrict the flow of oil through the steering valve, particularly to the oil grooves. Of course, any interference with fluid flow through the steering valve is highly undesirable.
As recognized in the Duffy patent noted above, the chamfered lands on the rotor and edges or corner breaks of the oil grooves reduce the turbulence in oil flow and provide an improved flow path from the pump means to the power assist chambers. Additionally, the chamfers reduce the hiss and noise associated with sharp or straight-edged surfaces. Still further, a better road feel results from use of the chamfered surfaces since the power assist is not abruptly altered but undergoes a smoother transition between neutral and assist positions.
It has been considered desirable to develop an accurate, cost effective manner of chamfering or forming alternative corner break arrangements in the valve sleeve of a power steering valve that overcomes the problems and shortcomings of prior art arrangements.